1. Field of the Invention
The present invention relates to an image sensor and a method for fabricating the same. More particularly, the invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor, and a method for fabricating the same, in which the fabricating cost is reduced by reducing photolithographic processes and yield is improved by obviating an alignment problem between color filter layers and microlenses.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that converts optical images to electrical signals. Semiconductor image sensors may be classified into charge coupled device (CCD) image sensors and CMOS image sensors.
The CMOS image sensor includes a photodiode area for sensing light and a CMOS logic circuit area for processing the sensed light to generate electrical signals. The photosensitivity characteristics of a photodiode may be dependent upon the dimensions of the photodiode. Generally, if the light-sensing area of the photodiode is large, the image sensor will have excellent photosensitivity characteristics.
To improve photosensitivity, it is necessary to increase the fill factor (i.e., the area occupied by the photodiode within the whole area of the image sensor). Alternatively, it may be necessary to change the path of incident light that may be directed toward an area other than the photodiode so as to converge light on the photodiode.
A microlens is generally used to converge light on the photodiode. A convex microlens made of material having good light transmittance characteristics is formed over the photodiode to refract a path of incident light, thereby transmitting and/or irradiating more light to the photodiode. In this case, light parallel to a light axis of the microlens is refracted by the microlens so that a focal point is formed at a certain position of or along the light axis.
CMOS image sensors may be classified into a three-transistor (3T) type, a four-transistor (4T) type, and a five-transistor (5T) type, depending on the number of transistors per unit pixel or photodiode. The 3T type CMOS image sensor includes a photodiode and three transistors. The 4T type CMOS image sensor includes a photodiode and four transistors. The 5T type CMOS image sensor includes a photodiode and five transistors. An equivalent circuit and a layout of a unit pixel of the 3T type CMOS image sensor will now be described with reference to FIGS. 1-3.
FIG. 1 is an equivalent circuit diagram illustrating a general 3T type CMOS image sensor. FIG. 2 is a layout illustrating a general 3T type CMOS image sensor. FIG. 3 is a sectional view illustrating the related art CMOS image sensor.
A unit pixel of the 3T type CMOS image sensor, as shown in FIG. 1, includes a photodiode PD and three NMOS transistors T1, T2 and T3. A cathode of the photodiode is connected to a drain of the first NMOS transistor T1 and a gate of the second NMOS transistor T2. Sources of the first and second NMOS transistors T1 and T2 are connected to a power line supplied with a reference voltage VR. A gate of the first NMOS transistor T1 is connected to a reset line supplied with a reset signal RST. Further, a source of the third NMOS transistor T3 is connected to a drain of the second NMOS transistor T2. The drain of the third NMOS transistor T3 is connected to a read circuit (not shown) through a signal line, and its gate is connected to a heat selection line supplied with a heat selection signal SLCT. Therefore, the first NMOS transistor T1 may be called a reset transistor Rx, the second NMOS transistor T2 may be called a drive transistor Dx, and the third NMOS transistor T3 may be called a selection transistor Sx.
In the unit pixel of the 3T type CMOS image sensor, as shown in FIG. 2, a photodiode 20 is formed in a wide portion of an active area 10, and gate electrodes 30, 40 and 50 of three transistors Rx, Dx, and Sx, respectively, are formed overlapping the remaining portion of the active area 10. In other words, the reset transistor Rx is defined in part by the gate electrode 30, the drive transistor Dx is defined in part by the gate electrode 40, and the selection transistor Sx is defined in part by the gate electrode 50. Impurity ions are implanted into the active area 10 of each transistor except portions below the gate electrodes 30, 40 and 50, to form the source and drain areas of each transistor Rx, Dx, and Sx. A power voltage Vdd is applied to the source areas between the reset transistor Rx and the drive transistor Dx. The drain of the selection transistor Sx is connected to a read circuit (not shown).
The gate electrodes 30, 40 and 50 are each provided with a pad at one end to enable connection to a signal line (not shown) from a respective external driving circuit (not shown).
Hereinafter, a related art CMOS image sensor having microlenses will be described with reference to FIG. 3. FIG. 3 is a sectional view illustrating the related art CMOS image sensor.
As shown in FIG. 3, the related art CMOS image sensor includes a sub (or substrate) layer 11 having photodiode areas and metal lines formed in a semiconductor substrate (not shown) to generate charges in response to incident light, a dielectric interlayer 12 formed on an entire surface of the sub layer 11, color filter layers 13 of red (R), green (G) and blue (B) formed on the dielectric interlayer 12 to respectively pass light of specific wavelengths to the photodiode areas, a planarization layer 14 formed on the color filter layers 13, and convex microlenses 15 having a certain curvature formed on the planarization layer 14 to converge light onto its corresponding color filter in color filter layers 13, thereby converging color-separated light on the photodiode areas.
Although not shown, a light-shielding layer may be formed in the dielectric interlayer 12 to prevent light from entering an area other than the photodiode areas. The CMOS image sensor may contain photogates, rather than photodiodes, for sensing light.
Each of the R, G and B color filter layers 13 is formed by a photolithography-etching process using a separate mask after depositing a corresponding photosensitive material.
Furthermore, the curvature and the height of the microlenses 15 are determined by considering various factors, such as the focal point of converged light. A photoresist is generally used to form the microlenses 15. The microlenses 15 are formed by deposition, patterning and reflow processes.
However, the related art CMOS image sensor and the method for fabricating the same have several problems.
First, since the color filter layers are formed below the microlenses, a misalignment problem may occur in operation of the image sensor if alignment of the microlenses 15 and the color filter layers 13 is beyond a limit range, thereby possibly resulting in a failure to display normal color.
Second, since the microlenses 15 are formed by reflowing the resist at a temperature between 150° C. and 200° C., they are susceptible to variations in temperature and thickness of the resist. Such factors may lead to alignment errors between the microlenses 15 and the color filter layers 13, thereby reducing yield.
Finally, since the color filter layers 13 and the microlenses 15 are formed respectively by separate photolithographic processes, the number of photolithographic processes is increased. This increases fabrication costs.